Handle exception when run on a Chef 11 server.

The current 'chef-server' cookbook that installs Erchef (Chef Server
version 11) does not create a 'chef' user.
inline chef_server?
extract chef_user_exists?
unify user & group selection, extract root_user & root_group
only use chef user when chef_server? and chef_user_exists?
eliminate use of removed variable
Move yola.rb out of the main Chef repo.

https://github.com/yola/Chef/blob/c31c95fb626525b09b6ec63e2318765534a240ff/cookbooks/chef-client/recipes/yola.rb
Merge remote-tracking branch 'remotes/josephholsten/fix/helper-erchef' into yola_cookbook
Merge pull request #1 from yola/yola_cookbook

Yolaify the chef-client cookbook